FS Distance from Camera settings to maintain constant line thickness
Short Answer: Not really possible right now.

Long Answer: There may be a workround.

First, Let's see what Range Min, Range Max, Value Min and Value Max mean:



As you can see, Range Min and Range Max actually indicate the distance between camera object and the nearest/fartherest point on the mesh bound. The Fill Range by Selection button can calculate those two values based on selected object. Value Min and Value Max are both thickness values in pixel.

However, these four parameters won't dynamically change with the relative distance between camera and extreme points on object. You have to drive them with that distance value:



Before (no driver):  After (with drivers): 

NOTE: The method above can be very limited because the distances to nearest and fartherest point can be different, which makes the expression less accurate. Anyway, assuming this is what you are going to make.